1. Field of the Invention
The present invention relates to an encoding noise reduction filter which is applicable to a decoded digital video signal obtained from a digital video signal encoding/decoding apparatus utilizing orthogonal transform processing.
2. Description of the Related Art
In recent years, as a result of advances that have been made in techniques for encoding of digital video signals, various types of apparatus have been proposed for processing a video signal to obtain an encoded video signal which can be transmitted or recorded as a data stream at a relatively low bit rate, e.g. for transmission or recording purposes, and for subsequently decoding the encoded video signal. In particular, methods have been proposed, for use with a moving-picture digital video signal encoding and decoding apparatus, whereby the quantization step size that is used in a quantization operation for the purpose of encoding the video signal is varied such as to maintain a constant bit rate for the encoded data. For example with an orthogonal transform type of encoding apparatus, the difference between each picture element luminance or chrominance value of the video signal and the corresponding value of the preceding frame of the video signal are mutually subtracted, and the resultant difference values are subjected to orthogonal transform processing (typically, Discrete Cosine Transform processing) to obtain respective transform coefficient values, which are quantized, with the quantization index values thus obtained being then encoded using a variable length encoding technique. The encoded data are then transferred through a buffer memory to be outputted as a bit stream, for transmission to a remote decoding apparatus, or for recording and subsequent decoding. Unless controlled, the rate of generation of encoded data will vary greatly between different frames, in accordance with the amount of activity occuring in the picture, i.e. if there are large differences between the contents of successive frames, then a large amount of encoded data will be generated for each frame. For that reason, the quantization step size may be controlled in accordance with the amount of data currently held in the buffer memory, in order to output the encoded data at a constant bit rate. Data which will be subsequently necessary for decoding purposes, including the quantization step size values, are also encoded and are transmitted to the decoding apparatus in the output bit stream from the encoding apparatus, multiplexed with the encoded data of the video signal.
Generally, each frame of the video signal is processed in the encoding apparatus as a set of 2-dimensional units of blocks of (picture element) values, with a specific value of quantization step size being determined for each block and used for all of that block., i.e. a fixed quantization step size is applied to the picture element data within each block. However the quantization step size can vary between successive blocks, in accordance with the state of the output buffer of the encoding apparatus as described above. Thus, successive values of quantization step size for respective blocks are outputted together with the code data corresponding to picture elements, in the bit stream produced from the encoding apparatus, to be used for dequantization processing in the decoding apparatus. The quantization step size can therefore vary within each frame of the video signal.
However with such a digital video signal encoding and decoding apparatus, when the quantization step size is large, a specific type of noise is produced in the display picture that is obtained from the decoded video signal. That noise is most conspicuous at the boundaries between different regions in picture conveyed by the video signal, and is conspicuous in the finally displayed picture. Such noise constitutes a significant problem.
Various types of encoding noise reduction methods have been proposed in the prior art for overcoming this problem. For example, such a video signal noise reduction method is described in Japanese Patent Laid-open No. 2-56179. With that method, basically, blurring of the picture expressed by the decoded video signal is produced by a filtering operation, with the filtering being selectively applied to picture elements which lie on the boundaries of picture regions which are visually uniform. More specifically, a decision as to whether or not filtering will be applied to a picture element (that is to say, applied to the luminance value of a picture element) is made in accordance with a judgement as to whether or not the picture element satisfies both of the following conditions (1) and (2):
(1) The picture element in question, which will be referred to as P.sub.a, is disposed at a position in the picture such as to be affected by the noise which is generated at a boundary between different regions in the picture, as indicated by a position relationship between the boundary and the position of picture element P.sub.a, obtained from information which expresses the boundary.
(2) The number n of picture elements P.sub.b which are disposed at the periphery of the picture element P.sub.a and which have a Laplacian value that is lower than a predetermined threshold value TH.sub.1 is greater than a predetermined threshold value TH.sub.n (where the specific meaning of the term "Laplacian value" as used in such an application will be described in detail hereinafter).
The noise filtering is selectively applied to only these picture elements P.sub.a for which the above conditions are satisfied. A conventional type of smoothing filtering (i.e. spatial low-pass filtering) can be used, for example.
However with such a prior art technique, the problem arises that the method of judging those picture elements that are to be subjected to the smoothing filtering is complex, and requires large amounts of processing. Moreover, due to the fact that no parameter is used in that judgement which expresses the overall quality of the picture expressed by the digital video signal, it is difficult to control the filter operation in accordance with picture quality.